Advanced electronic device structures using semiconductor structures and superlattices

ABSTRACT

Semiconductor structures and methods for forming those semiconductor structures are disclosed. For example, a semiconductor structure with a p-type superlattice region, an i-type superlattice region, and an n-type superlattice region is disclosed. The semiconductor structure can have a polar crystal structure with a growth axis that is substantially parallel to a spontaneous polarization axis of the polar crystal structure. In some cases, there are no abrupt changes in polarisation at interfaces between each region. At least one of the p-type superlattice region, the i-type superlattice region and the n-type superlattice region can comprise a plurality of unit cells exhibiting a monotonic change in composition from a wider band gap (WBG) material to a narrower band gap (NBG) material or from a NBG material to a WBG material along the growth axis to induce p-type or n-type conductivity.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.16/676,139, filed Nov. 6, 2019, which is a continuation of U.S. patentapplication Ser. No. 16/154,558, filed Oct. 8, 2018, which is acontinuation of U.S. patent application Ser. No. 15/853,379, filed Dec.22, 2017, which is a divisional of U.S. patent application Ser. No.15/601,890, filed May 22, 2017, which is a divisional of U.S. patentapplication Ser. No. 14/976,337, filed Dec. 21, 2015, which is acontinuation of International Patent Application numberPCT/IB2015/053203, filed May 1, 2015, which claims priority fromAustralian Provisional Patent Application number 2014902008 filed on May27, 2014 and entitled “Advanced Electronic Device Structures UsingSemiconductor Structures and Superlattices”, which are all incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates generally to advanced electronic devicestructures, typically using polar III-N semiconductor structures andsuperlattices. In particular, the invention relates, but is not limited,to semiconductor structures that are particularly suited to lightemitting diode (LED) structures, preferably for ultraviolet (UV) anddeep UV (DUV) LEDs operating in the wavelength range of 190-280 nm.Although the invention is primarily described with reference to UV andDUV LEDs, it will be appreciated that these are preferred applicationsonly, and that other applications may be apparent to persons skilled inthe art.

BACKGROUND TO THE INVENTION

Wide band gap semiconductors, such as Aluminium-Gallium-Nitride (AlGaN)have a well known limitation of poor conductivity p-type or n-typecreation, especially for p-type materials using an impurity atomsubstitutional doping method. At present the highest p-type acceptordensity is achieved in p-GaN, with a substantial reduction in availablehole concentrations with increasing band gap as the aluminium molefraction is increased. This limits DUV LED development in relation toachieving electronic grade high n-type and p-type donor and acceptorconcentrations in sufficiently wide band gap compositions of, forexample, AlGaN, and more generally AlGaInN semiconductors.

DUV LEDs typically achieve light emission by advantageous spatialrecombination of electrons and holes within a direct band gapcrystalline structure. They fundamentally operate as a two electricalport device and are built from at least one of a p-i-n or p-nheterojunction diode with the emission region confined substantially toa region between the p-type and n-type regions. If the emission energyis smaller than the bandgap energy of at least one of the p-type andn-type cladding layers comprising the diode, then the photocarriergenerated light can escape from within the device.

SUMMARY OF INVENTION

P-type doping limitations in III-N device development are one of thegreatest restraints in developing commercially viable DUV LEDs.Accordingly, there is a need for improved impurity dopants, especiallyfor p-type characteristics in III-N materials.

In one form, although it need not be the only or indeed the broadestform, there is provided a method of forming a p-type or n-typesemiconductor structure. The method comprises:

-   -   growing along a growth axis a semiconductor having a polar        crystal structure, the growth axis being substantially parallel        to a spontaneous polarization axis of the crystal structure; and    -   changing the composition of the semiconductor monotonically from        a wider band gap (WBG) material to a narrower band gap (NBG)        material or from a NBG material to a WBG material along the        growth axis to induce p-type or n-type conductivity.

Preferably, the composition of the semiconductor comprises: at least twotypes of metal atom cation; and a non-metal atom anion.

Preferably, the non-metal atom anion is nitrogen or oxygen.

Preferably, changing the composition of the semiconductor comprises:changing a molar fraction of one or more of the at least two types ofmetal atom cation in the composition along the growth axis.

Preferably, the p-type conductivity is induced by:

-   -   growing the semiconductor with a cation-polar crystal structure        and changing the composition of the semiconductor monotonically        from a WBG material to a NBG material along the growth axis; or    -   growing the semiconductor with an anion-polar crystal structure        and changing the composition of the semiconductor monotonically        from a NBG material to a WBG material along the growth axis.

Preferably, the n-type conductivity is induced by:

-   -   growing the semiconductor with a cation-polar crystal structure        and changing the composition of the semiconductor monotonically        from a NBG material to a WBG material along the growth axis; or    -   growing the semiconductor with an anion-polar crystal structure        and changing the composition of the semiconductor monotonically        from a WBG material to a NBG material along the growth axis.

Preferably, the polar crystal structure is a polar wurtzite crystalstructure.

Preferably, the composition of the semiconductor is changed in acontinuous manner or a stepwise manner along the growth axis.

Suitably, the composition of the semiconductor is selected fromgroup-III metal nitride compositions.

Suitably, the composition of the semiconductor is selected from thefollowing: aluminium gallium nitrides (Al_(x)Ga_(1−x)N) where 0≤x≤1;aluminium gallium indium nitrides (Al_(x)Ga_(y)In_(1−x−y)N) where 0≤x≤1,0≤y≤1 and 0≤(x+y)≤1; and magnesium zinc oxides (Mg_(x)Zn_(x−1)O) where0≤x≤1.

Suitably, the method further comprises: including impurity dopants inthe composition of the semiconductor to enhance the induced p-type orn-type conductivity.

In another form, there is provided a method of forming a p-type orn-type semiconductor superlattice comprising a plurality of unit cellseach comprising at least two distinct layers formed of a substantiallysingle crystal semiconductor. The method comprises:

-   -   growing along a growth axis the superlattice having a polar        crystal structure, the growth axis being substantially parallel        to a spontaneous polarization axis of the crystal structure; and    -   changing an average composition of the unit cells of the        superlattice monotonically from an average composition        corresponding to a wider band gap (WBG) material to an average        composition corresponding to a narrower band gap (NBG) material        or from an average composition corresponding to a NBG material        to an average composition corresponding to a WBG material along        the growth axis to induce p-type or n-type conductivity.

Preferably, the p-type conductivity is induced by:

-   -   growing the superlattice with a cation-polar crystal structure        and changing the average composition of the unit cells        monotonically from an average composition corresponding to a WBG        material to an average composition corresponding to a NBG        material along the growth axis; or    -   growing the superlattice with an anion-polar crystal structure        and changing the average composition of the unit cells        monotonically from an average composition corresponding to a NBG        material to an average composition corresponding to a WBG        material along the growth axis.

Preferably, the n-type conductivity is induced by:

-   -   growing the superlattice with a cation-polar crystal structure        and changing the average composition of the unit cells        monotonically from an average composition corresponding to a NBG        material to an average composition corresponding to a WBG        material along the growth axis; or    -   growing the superlattice with an anion-polar crystal structure        and changing the average composition of the unit cells        monotonically from an average composition corresponding to a WBG        material to an average composition corresponding to a NBG        material along the growth axis.

Preferably, the anion-polar crystal structure is a nitrogen-polarcrystal structure or an oxygen-polar crystal structure.

Preferably, the cation-polar crystal structure is a metal-polar crystalstructure.

Preferably, the average composition of the unit cells is changed in acontinuous manner or a stepwise manner along the growth axis.

Suitably, the average composition of the unit cells is changed bychanging a thickness of one or more of the at least two distinct layersof the unit cells.

Suitably, a thickness of the unit cells is constant along the growthaxis.

Suitably, a composition of one or more of the at least two distinctlayers of the unit cells is selected from the following:

-   -   gallium nitride (GaN);    -   aluminium nitride (AlN);    -   aluminium gallium nitride (Al_(x)Ga_(1−x)N) where 0≤x≤1;    -   boron aluminium nitride B_(x)Al_(1−x)N where 0≤x≤1; and        aluminium gallium indium nitride (Al_(x)Ga_(y)In_(1−x−y)N) where        0≤x≤1, 0≤y≤1 and 0≤(x+y)≤1.

Suitably, a composition of one or more of the at least two distinctlayers of the unit cells is selected from the following:

-   -   magnesium oxide (MgO);    -   zinc oxide (ZnO); and    -   magnesium zinc oxide (Mg_(x)Zn_(1−x)O) where 0≤x≤1.

Preferably, the at least two distinct layers of each unit cell each havea thickness that is less than the de Broglie wavelength of a chargecarrier in the respective layer.

Preferably, the at least two distinct layers of each unit cell each havea thickness that is less than or equal to a critical layer thicknessrequired to maintain elastic strain.

Suitably, the method further comprises: including impurity dopants inone or more of the least two distinct layers of each unit cell toenhance the induced p-type or n-type conductivity.

In another form, there is provided a method of forming a complexsemiconductor structure. The method comprises: forming two or morecontiguous semiconductor structures and/or semiconductor superlattices,wherein the semiconductor structures and/or semiconductor superlatticesare each formed according to a method previously described herein.

Suitably, the method of forming a complex semiconductor structurefurther comprises flipping the polarity-type of the material between twoof the two or more contiguous semiconductor structures and/orsemiconductor superlattices.

Suitably, a first of the two or more contiguous semiconductor structuresand/or semiconductor superlattices has a larger change in compositionalong the growth axis and a second of the two or more contiguoussemiconductor structures and/or semiconductor superlattices has asmaller change in composition along the growth axis.

Suitably, a first of the two or more contiguous semiconductor structuresand/or semiconductor superlattices induces a heavy p-type conductivity,and a second of the two or more contiguous semiconductor structuresand/or semiconductor superlattices induces a light p-type conductivity.

In another form, there is provided a method of forming a light emittingdiode (LED) structure. The method comprises:

-   -   growing along a growth axis, between a wider band gap (WBG)        n-type region and a narrower band gap (NBG) p-type region, a        semiconductor structure having a polar crystal structure in        which a spontaneous polarization axis is parallel to the growth        axis, the semiconductor structure comprising a semiconductor        that changes in composition monotonically from a wider band gap        (WBG) material adjacent the WBG n-type region to a narrower band        gap (NBG) material adjacent the NBG p-type region.

In another form, there is provided a method of forming a light emittingdiode (LED) structure. The method comprises:

-   -   growing along a growth axis, between a wider band gap (WBG)        n-type region and a narrower band gap (NBG) p-type region, a        superlattice comprising a plurality of unit cells each        comprising at least two distinct layers formed of a        substantially single crystal semiconductor, the superlattice        having a polar crystal structure in which a spontaneous        polarization axis is parallel to the growth axis, and the unit        cells changing in average composition monotonically from an        average composition corresponding to a wider band gap (WBG)        material in a unit cell adjacent the WBG n-type region to an        average composition corresponding to a narrower band gap (NBG)        material in a unit cell adjacent the NBG p-type region.

Preferably, a buffer or dislocation filter region is grown on asubstrate preceding the WBG n-type region or NBG p-type region.

Suitably, the substrate is selected as a sapphire (Al₂O₃) substrate oran aluminium nitride (AlN) substrate if the WBG n-type region is grownbefore the NBG p-type region; or the substrate is selected as a siliconsubstrate or a gallium nitride (GaN) substrate if the NBG p-type regionis grown before the WBG n-type region.

In another form, there is provided a p-type or n-type semiconductorstructure formed in accordance with a method previously describedherein.

In another form, there is provided a p-type or n-type semiconductorsuperlattice formed in accordance with a method previously describedherein.

In another form, there is provided a complex semiconductor structureformed in accordance with a method previously described herein.

In another form, there is provided a light emitting diode (LED)structure formed in accordance with a method previously describedherein.

In another form, there is provided a p-type or n-type semiconductorstructure having a polar crystal structure with a growth axis that issubstantially parallel to a spontaneous polarization axis of the polarcrystal structure, the semiconductor structure changing in compositionmonotonically from a wider band gap (WBG) material to a narrower bandgap (NBG) material or from a NBG material to a WBG material along thegrowth axis to induce p-type or n-type conductivity.

In another form, there is provided a p-type or n-type semiconductorsuperlattice comprising a plurality of unit cells each comprising atleast two distinct layers formed of a substantially single crystalsemiconductor, the superlattice having a polar crystal structure with agrowth axis being substantially parallel to a spontaneous polarizationaxis of the polar crystal structure, the average composition of the unitcells of the superlattice changing monotonically from an averagecomposition corresponding to a wider band gap (WBG) material to anaverage composition corresponding to a narrower band gap (NBG) materialor from an average composition corresponding to a NBG material to anaverage composition corresponding to a WBG material along the growthaxis to induce p-type or n-type conductivity.

In another form, there is provided a complex semiconductor structurecomprising two or more contiguous semiconductor structures and/orsemiconductor superlattices previously described herein.

In another form, there is provided a light emitting diode (LED)structure comprising: a semiconductor structure formed between a widerband gap (WBG) n-type region and a narrower band gap (NBG) p-typeregion, the semiconductor structure having a polar crystal structure inwhich a spontaneous polarization axis is parallel to the growth axis ofthe crystal structure, and the semiconductor structure comprising asemiconductor that changes in composition monotonically from a widerband gap (WBG) material adjacent the WBG n-type region to a narrowerband gap (NBG) material adjacent the NBG p-type region.

In another form, there is provided a light emitting diode (LED)structure comprising: a superlattice formed between a wider band gap(WBG) n-type region and a narrower band gap (NBG) p-type region, thesuperlattice comprising a plurality of unit cells each comprising atleast two distinct layers formed of a substantially single crystalsemiconductor, the superlattice having a polar crystal structure inwhich a spontaneous polarization axis is parallel to the growth axis ofthe crystal structure, and the unit cells changing in averagecomposition monotonically from an average composition corresponding to awider band gap (WBG) material in a unit cell adjacent the WBG n-typeregion to an average composition corresponding to a narrower band gap(NBG) material in a unit cell adjacent the NBG p-type region.

In another form, there is provided a semiconductor structure comprising:a p-type superlattice region;

-   -   an i-type superlattice region; and    -   an n-type superlattice region;    -   wherein at least one of the p-type superlattice region, the        i-type superlattice region and the n-type superlattice region        comprises a monotonic change in average composition from an        average composition corresponding to a wider band gap (WBG)        material to an average composition corresponding to a narrower        band gap (NBG) material, or from an average composition        corresponding to a NBG material to an average composition        corresponding to a WBG material, such that there are no abrupt        changes in polarisation at the interfaces between each region.

Preferably, the semiconductor structure further comprises a p-type GaNregion adjacent the p-type superlattice region.

Further features and advantages of the present invention will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separateviews, together with the detailed description below, are incorporated inand form part of the specification, and serve to further illustrateembodiments of concepts that include the claimed invention, and explainvarious principles and advantages of those embodiments.

FIG. 1 illustrates a sectional view through slabs of wurtziticgroup-III-metal-nitride crystals with either a metal polar orientation(on the left) or a nitrogen polar orientation (on the right).

FIG. 2 illustrates a periodic structure for a metal-polar wurtzitestructure representing an ordered bulk alloy or bilayered superlatticeof equal AlN and GaN proportions.

FIG. 3A illustrates a structure having a linear gradient region of bulkmaterials.

FIG. 3B illustrates a bandgap diagram for the structure illustrated inFIG. 3A.

FIG. 3C illustrates spatial variation of induced piezoelectric chargedensity for the structure illustrated in FIG. 3A.

FIG. 3D illustrates spatial variation of induced pyroelectric chargedensity for the structure illustrated in FIG. 3A.

FIG. 3E illustrates spatial band structure for the structure illustratedin FIG. 3A.

FIG. 3F illustrates estimated spatial variation of the areal electronconcentration and the areal heavy-hole (HH) concentrations for thestructure illustrated in FIG. 3A.

FIG. 3G illustrates a detail plot of zone centre variation in the lowestenergy conduction band edge for the structure illustrated in FIG. 3A.

FIG. 3H illustrates a detail plot for zone centre variation in the threehighest lying valence band edges.

FIG. 3I illustrates full spatial zone centre bandstructure for thestructure illustrated in FIG. 3A.

FIG. 3J illustrates a detail plot of estimated conduction contrastingmetal-polar or nitrogen-polar orientation for the structure illustratedin FIG. 3A.

FIG. 3K illustrates a detail plot of estimated valence heavy-hole (HH)contrasting metal-polar or nitrogen-polar orientation for the structureillustrated in FIG. 3A.

FIG. 3L illustrates an optical rectification effect for a linearlygraded bandgap, such as for the structure illustrated in FIG. 3A,showing a preferred direction for outcoupling of photons from thestructure.

FIG. 3M illustrates an XRD simulation of the structure illustrated inFIG. 3A.

FIG. 4A illustrates a structure having a stepwise gradient region ofbulk materials.

FIG. 4B illustrates a bandgap diagram for the structure illustrated inFIG. 4A.

FIG. 4C illustrates a spatial dependence full bandstructure for thestructure illustrated in FIG. 4A.

FIG. 4D illustrates zone centre conduction band variation as a functionof growth distance for the structure illustrated in FIG. 4A.

FIG. 4E illustrates zone centre valence band edge variation as afunction of growth distance for HH, LH, and CH bands structureillustrated in FIG. 4A.

FIG. 4F illustrates spatial variation of induced piezoelectric chargedensity for the structure illustrated in FIG. 4A.

FIG. 4G illustrates spatial variation of induced pyroelectric chargedensity structure illustrated in FIG. 4A.

FIG. 4H illustrates electron and heavy-hole (HH) carrier concentrationsgenerated within the structure illustrated in FIG. 4A.

FIG. 5A illustrates a structure having a not-intentionally dopedlinearly chirped superlattice composition with a fixed period.

FIG. 5B illustrates a bandgap diagram for the structure illustrated inFIG. 5A.

FIG. 5C illustrates a variation of the structure illustrated in FIG. 5A.

FIG. 5D illustrates a full zone centre spatial bandstructure of thestructure illustrated in FIG. 5C.

FIG. 6 illustrates a P-UP LED structure.

FIG. 7 illustrates a P-DOWN LED structure.

FIG. 8 illustrates a spatial band energy plot for a semi-infinitesuperlattice built from two repetitions of AlN/GaN.

FIG. 9 illustrates a valence band dispersion for an intentionallyordered superlattice comprising a binary bilayered superlattice.

FIG. 10A illustrates a spatial band structure for a linearly chirpedsuperlattice with piezoelectric and pyroelectric fields absent.

FIG. 10B illustrates a spatial band structure for a linearly chirpedsuperlattice with polarization fields applied.

FIG. 11A illustrates an electron and heavy-hole valence quantized energyfor a linearly chirped superlattice.

FIG. 11B illustrates a confined spatial wavefunction for a linearlychirped superlattice.

FIG. 12A illustrates a stack for generating electrical and opticalportions of a p-n diode according to some embodiments.

FIG. 12B illustrates thicknesses of GaN and AlN layers in the unit cellof a superlattice to achieve a desired average alloy composition.

FIG. 12C illustrates the average alloy content as a function of periodsalong the growth axis for each of the n:SL and the i:CSL in the stack inFIG. 12A.

FIG. 12D shows the calculated spatial energy band structure of theconduction and heavy-hole bands of the stack in FIG. 12A.

FIG. 12E shows the electron and hole carrier concentrations induced inthe stack of FIG. 12A.

FIG. 12F shows the calculated lowest energy n=1 quantized electronspatial wavefunctions within the stack of FIG. 12A.

FIG. 12G shows the calculated lowest energy n=1 quantized heavy-holespatial wavefunctions within the stack of FIG. 12A

FIG. 12H shows the calculated overlap integrals between the lowestenergy n=1 quantized electron and heavy-hole spatial wavefunctionswithin the stack of FIG. 12A.

FIG. 12I shows the calculated optical emission spectrum for the stack ofFIG. 12A

FIG. 13A illustrates a stack for generating electrical and opticalportions of a p-i-n diode according to some embodiments.

FIG. 13B shows the calculated spatial energy band structure of theconduction and heavy-hole bands of the stack in FIG. 13A.

FIG. 13C shows the electron and hole carrier concentrations induced inthe stack of FIG. 13A.

FIG. 13D shows the calculated overlap integrals between the lowestenergy n=1 quantized electron and heavy-hole spatial wavefunctionswithin the stack of FIG. 13A.

FIG. 13E shows the calculated optical emission spectrum for the stack ofFIG. 13A.

FIG. 14 illustrates an example two port LED structure.

FIG. 15 illustrates a gradient pattern growth sequence for a chirpedperiod and constant x_(ave) superlattice.

FIG. 16 illustrates polarization type flipping of a wurtzite orderedAlN/GaN superlattice using an interlayer chosen from atomic species oftype X2+ or X4+.

FIG. 17 illustrates a flow diagram of a method of forming asemiconductor structure.

FIG. 18A illustrates a semiconductor structure.

FIG. 18B illustrates a band energy structure for a device according tothe semiconductor structure of FIG. 18A.

FIG. 18C illustrates a band energy structure for another deviceaccording to the semiconductor structure of FIG. 18A.

FIG. 18D illustrates a band energy structure for a device according tothe semiconductor structure of FIG. 18A.

FIG. 18E illustrates a band energy structure for a device according tothe semiconductor structure of FIG. 18A.

FIG. 18F illustrates a band energy structure for a device according tothe semiconductor structure of FIG. 18A.

FIG. 18G illustrates a band energy structure for a device according tothe semiconductor structure of FIG. 18A.

FIG. 18H illustrates a band energy structure for a device according tothe semiconductor structure of FIG. 18A.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

The components in the drawings have been represented where appropriateby conventional symbols, showing only those specific details that arepertinent to understanding the embodiments of the present invention soas not to obscure the disclosure with details that will be readilyapparent to those of ordinary skill in the art having the benefit of thedescription herein.

DETAILED DESCRIPTION OF THE DRAWINGS

Generally, the present invention relates to growth of a semiconductorstructure or a semiconductor superlattice that has a polar crystalstructure, such as a wurtzite polar crystal structure, and is grownalong a growth axis (growth direction), with a spontaneous polarizationaxis of the crystal structure substantially parallel to the growth axis.Such polar crystal structures are typically characterized as having acrystal lattice possessing a non-inversion symmetry, a spontaneouspolarization axis and a distinct growth orientation when deposited alonga polarization axis.

The superlattice comprises a plurality of unit cells each comprising atleast two distinct layers formed of a substantially single crystalsemiconductor. In preferred embodiments, the semiconductor superlatticeis a short period superlattice (SPSL). Properties of the semiconductorstructure or the semiconductor superlattice are engineered by changing acomposition of a semiconductor in the semiconductor structure, or a bulkor an average composition of a unit cell of the superlattice,monotonically along the growth axis. Such a change in composition isalso referred to herein as a grading pattern or grading region. Forexample, the composition of the semiconductor in the semiconductorstructure or the average composition of the unit cells is changed in acontinuous manner or a stepwise manner along the growth axis.

In preferred embodiments, the composition of the semiconductor comprisesat least one type, and preferably at least two types, of metal atomcation and a non-metal atom anion. However, in some embodiments, thecomposition of the semiconductor comprises more than one type ofnon-metal atom anion. For example, the non-metal atom anion can benitrogen or oxygen. In some embodiments, the composition of thesemiconductor is changed by changing a molar fraction of one or more ofthe at least two types of metal atom cation in the composition along thegrowth axis. In some embodiments, the average composition of the unitcells in the superlattice is changed by changing a thickness of one ormore of the at least two distinct layers of the unit cells. In preferredembodiments, the at least two distinct layers of each unit cell eachhave a thickness that is less than the de Broglie wavelength of a chargecarrier, for example, an electron or a hole, in the respective layer. Inpreferred embodiments, the at least two distinct layers of each unitcell also each have a thickness that is less than or equal to a criticallayer thickness required to maintain elastic strain.

In preferred embodiments, the composition of the semiconductor structureis changed monotonically from a wider band gap (WBG) material to anarrower band gap (NBG) material or from a NBG material to a WBGmaterial along the growth axis. This can induce p-type or n-typeconductivity and make the semiconductor structure p-type or n-type.

For example, p-type conductivity can be induced by growing thesemiconductor with a cation-polar crystal structure, such as ametal-polar crystal structure, and changing the composition of thesemiconductor monotonically from a WBG material to a NBG material alongthe growth axis. Alternatively, p-type conductivity can be induced bygrowing the semiconductor with an anion-polar crystal structure, such asa nitrogen-polar crystal structure or an oxygen-polar crystal structure,and changing the composition of the semiconductor monotonically from aNBG material to a WBG material along the growth axis.

For example, n-type conductivity can be induced by growing thesemiconductor with a cation-polar crystal structure, such as ametal-polar crystal structure, and changing the composition of thesemiconductor monotonically from a NBG material to a WBG material alongthe growth axis. Alternatively, n-type conductivity can be induced bygrowing the semiconductor with an anion-polar crystal structure, such asa nitrogen-polar crystal structure or an oxygen-polar crystal structure,and changing the composition of the semiconductor monotonically from aWBG material to a NBG material along the growth axis.

Similarly, in preferred embodiments, a semiconductor superlattice isengineered, for example to induce p-type or n-type conductivity, bychanging an average composition of the unit cells of the superlatticemonotonically from an average composition corresponding to a wider bandgap (WBG) material to an average composition corresponding to a narrowerband gap (NBG) material or from an average composition corresponding toa NBG material to an average composition corresponding to a WBG materialalong the growth axis.

For example, p-type conductivity can be induced by growing thesuperlattice with a cation-polar crystal structure, such as ametal-polar crystal structure, and changing the average composition ofthe unit cells monotonically from an average composition correspondingto a WBG material to an average composition corresponding to a NBGmaterial along the growth axis. Alternatively, p-type conductivity canbe induced by growing the superlattice with an anion-polar crystalstructure, such as a nitrogen-polar crystal structure or an oxygen-polarcrystal structure, and changing the average composition of the unitcells monotonically from an average composition corresponding to a NBGmaterial to an average composition corresponding to a WBG material alongthe growth axis.

For example, n-type conductivity can be induced by growing thesuperlattice with a cation-polar crystal structure, such as ametal-polar crystal structure, and changing the average composition ofthe unit cells monotonically from an average composition correspondingto a NBG material to an average composition corresponding to a WBGmaterial along the growth axis. Alternatively, n-type conductivity canbe induced by growing the superlattice with an anion-polar crystalstructure, such as a nitrogen-polar crystal structure or an oxygen-polarcrystal structure, and changing the average composition of the unitcells monotonically from an average composition corresponding to a WBGmaterial to an average composition corresponding to a NBG material alongthe growth axis.

A complex semiconductor structure, for example, for use in asemiconductor device, such as an LED, can be formed from two or moresemiconductor structures and/or semiconductor superlattices. Forexample, a complex semiconductor structure can be formed by stacking twoor more semiconductor structures and/or semiconductor superlatticescontiguously on top of one another. If necessary, a polarity-type of thematerial can be flipped between two of the two or more contiguoussemiconductor structures and/or semiconductor superlattices.

A light emitting diode (LED) structure can be formed using a gradingregion, for example, as an i-type region, between a WBG n-type regionand a NBG p-type region and/or by using the grading region as an n-typeregion or a p-type region. In such a way, a light emitting diode (LED)structure can be formed such that there are no abrupt changes inpolarisation at the interfaces between each region.

In preferred embodiments, the semiconductor structure or semiconductorsuperlattice is formed from Group-III metal nitride (III-N) compounds,for example, gallium nitride (GaN), aluminium nitride (AlN), aluminiumgallium nitride (Al_(x)Ga_(1−x)N) where 0≤x≤1, boron aluminium nitrideB_(x)Al_(1−x)N where 0≤x≤1; or aluminium gallium indium nitride(Al_(x)Ga_(y)In_(1−x−y)N) where 0≤x≤1, 0≤y≤1 and 0≤(x+y)≤1. However, thesemiconductor structure or semiconductor superlattice can be formed ofother compounds, for example, magnesium oxide (MgO), zinc oxide (ZnO)and magnesium zinc oxide (Mg_(x)Zn_(1−x)O) where 0≤x≤1. In someembodiments, impurity dopants are also included in the composition ofthe semiconductor or in one or more of the least two distinct layers ofeach unit cell to enhance the induced p-type or n-type conductivity.

III-N compounds readily crystallize in stable hexagonal crystalstructures classified as Wurtzite-type structures. These III-N wurtzitestructures can be deposited on a substrate. For example, they can bedeposited epitaxially on an atomically flat two-dimensional hexagonalcrystal substrate surface that may be formed by an advantageouslyterminated plane of a 3-dimensional bulk crystal. Ideally, the substrateis atomically flat and composed of the topmost atomic layer ofhomogenous atomic species. Furthermore, the surface layer atom bondingtype and in-plane lattice constant is commensurate with forming latticematched or pseudomorphic epitaxial growth.

A distinguishing property of wurtzitic III-N crystals is the highlypolar nature of the metal-nitrogen bond which forces asymmetry in thewurtzite crystal structure perpendicular to the substrate surface plane(often known as ‘crystal-plane’ or ‘c-plane’). Depending upon the firstatomic species (e.g. nitrogen or metal) forming the epitaxial layer on anon-native crystal surface, there exists two unique and physicallydistinguishable wurtzite crystal orientations as shown in FIG. 1. Thetwo crystal orientations shown in FIG. 1 are known as metal-polar 100 ornitrogen-polar 120 having metal-polar epitaxy 102 or nitrogen polarepitaxy 122, respectively.

The polarization effect within the crystal planes can be utilised tomanipulate different properties in heterostructures for the presentinvention. Alternatively, wurtzite III-N (wz-III-N) bulk-like substratesor thick III-N film can be formed having a preferred crystallinepolarity orientation relative to a direction perpendicular to thec-plane.

An intentionally ordered pseudo-alloy can be formed using accuratelycontrolled deposition processes to form monolayered (ML) or fractionalmonolayered films of, for example, GaN 210 and AlN 220 on a substrate200 as shown in FIG. 2. An ideal superlattice formed using repetitiveunit cells of 0.5 ML GaN 210 to 0.5 ML AlN 220 can form an ideal orderedAl_(0.5)Ga_(0.5)N alloy as illustrated. It will be appreciated, however,that other layer thicknesses of GaN and AlN comprising the unit cellcould also be deposited. The structure illustrated in FIG. 2 isconsidered ideal, exhibiting superior piezoelectric and pyroelectricpolarization compared to an equivalent randomly arranged metal cationsin a bulk alloy.

FIG. 3A illustrates a semiconductor structure in the form of a diode 300formed with bulk-like materials. The diode 300 has, in order along agrowth axis 310, a lower wurtzite metal layer 320 with a metal-polargrowth including an n-type Al_(0.8) Ga_(0.2)N WBG emitter 330, agradient region in the form of an intrinsic Al_(x)Ga_(1−x)N alloy 340with a linear variation of composition along the growth axis thatlinearly transitions from the WBG wurtzite metal layer 320 to a NBGp-type GaN contact layer 350, and finally, an upper wurtzite metal layer360. The lower wurtzite metal layer 320 and the upper wurtzite metallayer 360 are effective ohmic metal contacts to form two electricalcontacts for the diode 300.

FIG. 3B illustrates a spatial composition or bandgap energycorresponding to the diode 300 of FIG. 3A, showing how the bandgaptransitions linearly over a gradient region 342 from a WBG material 332to a NBG material 352. Indicator lines 343 illustrate example variationsthat may be achieved in the gradient region 342 for continuoustransitions that are non-linear. As will be appreciated, the WBGmaterial 332, gradient region 342, and NBG material 352 in FIG. 3Bcorrespond to the WBG emitter 330, linearly graded alloy 340, and theNBG contact layer 350 of FIG. 3A.

From an understanding of the fundamental behaviour of polarizationfields within wz-III-N materials, the induced piezoelectric (due tolattice deformation) and pyroelectric (due to spontaneous polarization)charge profiles along the growth axis can be determined for the diode300 illustrated in FIG. 3A, as shown in FIGS. 3C and 3D, respectively.

For a linear Al % variation in Al_(x(z))Ga_(1−x(z))N having compositionprofile given by x(z) in the transition region, the piezoelectric andpyroelectric charge densities vary as a function of z with diminishingcharge approaching the NBG p-GaN layer.

The two cases of metal-polar and nitrogen-polar epitaxial structuresdeposited along the spontaneous polarization axis, being the c-axis,generate contrasting polarisation fields. This correlation of chargesign with film polarity-type is used advantageously to improve theelectron and/or hole carrier concentration.

While not obvious, the implications of such an areal charge density,which varies along the growth axis 310, is that warping of theconduction and valence band edges effectively ‘pins’ or shifts theconduction band or valence band edge to the Fermi-level depending uponthe growth polarity of the film. The variation in x(z) produces acommensurate variation in the position dependent strain tensor, due tothe difference in the in-plane lattice constant for each materialcomposition. This change in the bulk crystal lattice constant produces abi-axial strain and is assumed to generate an elastic deformation of thecrystal, and thus induces a piezoelectric charge. In these examples theepitaxial stack is assumed to be deposited pseudomorphically on a thickand relaxed AlN buffer, and thus the stack is strained to the freestanding bulk in-plane lattice constant of AlN. Other buffer layers andlattice constants are also possible. However, it is the critical layerthickness (CLT) which limits the thickness to which a lattice mismatchedmaterial can be pseudomorphically deposited. This limitation can beameliorated using a superlattice comprising unit cells with each unitcell comprising at least two layers of lattice mismatched compositions,where the thickness of each layer is below the CLT of that layer withrespect to the buffer in-plane lattice constant. That is, a superlatticecan improve the ability to form large changes in average compositionspatially according to embodiments of the present invention.

FIG. 3E illustrates full spatial (k=0) energy band structure for thediode 300 illustrated in FIG. 3A showing the effect of a linearcompositional variation of a WBG to a NBG transition provided bylinearly graded alloy 340. The not-intentionally doped compositionallyvaried region is sandwiched between n-type WBG and p-type NBG slabs. Theinduced depletion region is localised toward the n-type WBG ori-compositionally varied region producing Fermi level pinning of thevalence band for the case of a metal-polar oriented growth.

FIG. 3F illustrates spatial variation of the areal electronconcentration and the areal heavy-hole concentrations for the diode 300illustrated in FIG. 3A. The linear spatial variation in the AlGaN alloycomposition x(z) induces a large hole carrier concentration in theotherwise not-intentionally doped region. Holes would therefore besupplied by the p-GaN contact region and transported into the inducedp-type region. The depletion region extends into the n-type WBG regionindicating that the induced p-type behaviour of the linear compositionregion is higher than the intentional ionized donor concentration.

FIGS. 3G and 3H illustrate plots of the epitaxial structure of the diode300 of FIG. 3A for the zone centre spatial variation in the lowestenergy conduction band edge EC(k=0,z) (FIG. 3G) and the three highestlying valence band edges E_(v)(k=0,z), where v=HH, LH & CH (FIG. 3H).FIG. 3G shows the p-i-n diode formed using AlGaN but for the case ofpiezoelectric and pyroelectric charges set to zero. FIG. 3H shows theresult for comparison with the piezoelectric and pyroelectric chargestaken into account. It can be seen that the polarization charges shouldbe accounted for when designing polar devices. Note also that thevariation in Al % in Al_(x(z))Gal_(x(z))N due to the x(z) for the range0.0≤x(z)≤0.8 for bulk-like material will have a cross-over in the lowestenergy valence band at k=0 occurring at x(z)˜0.65. For values lessx<0.65, the material will have the heavy-hole valence band as thedominant hole type, whereas for x>0.65, the crystal field split valenceband will dominate. FIG. 3I illustrates a full spatial zone centre bandstructure of diode 300 of FIG. 3A for two contrasting cases of epitaxialgrowth, namely with metal-polar or nitrogen-polar orientation relativeto the grown axis 310. The result shows that a composition transitionfrom a WBG to a NBG along the growth axis induces a p-type behaviour formetal-polar growth or an n-type behaviour for nitrogen-polar growth.

FIGS. 3J and 3K illustrate plots of conduction and valence heavy-holespatial zone-centre bandstructure, respectively, of the diode 300 ofFIG. 3A for two contrasting cases of epitaxial growth, namelymetal-polar or nitrogen-polar orientation relative to the grown axis310. The illustrated effect of film polarity dramatically influences theelectronic behaviour of the device. For a metal-polar film thenot-intentionally doped linear alloy composition variation x(z) inducesp-type behaviour, whereas the nitrogen-polar orientation induces n-typebehaviour. The respective depletion regions are contrasted and dictatethe device operation. This fundamental effect can be used advantageouslyfor semiconductor structures, particularly LED structures.

FIG. 3L illustrates an optical rectification effect for a linearlygraded bandgap region, as issued in diode 300, schematically showing apreferred direction for outcoupling of photons from the structure.Electrons and holes injected into the recombination region can generatea broad optical emission spectrum due to the spatially dependentbandgap. Preferably, the recombination region is spatially coincidentwith the depletion region of the diode. High energy photons (i.e. shortwavelength λ_(S)) generated within the depletion region with energy lessthan the n-type WBG region can propagate with low-loss through then-type WBG material 330 and substrate 320, whereas forward propagatingphotons will be reabsorbed within the spatially decreasing bandgaptoward the p-NBG material 350. Longer wavelength photons (λ_(L)) aretherefore preferentially emitted through the topmost NBG layer. Thelarge and asymmetrical built-in conduction band potential impedes freetransport J_(e)(z) of electrons across the structure. This photonrecycling through absorption process can improve the p-type regionperformance.

FIG. 3M shows an X-ray diffraction (XRD) estimate of the diode 300 ofFIG. 3A illustrating gradient region characteristics 344 of the linearlygraded alloy. XRD analysis, particularly when looking at the gradientregion characteristics 344, can be used to confirm epitaxial grownsequences and tailor the spatial composition variation.

FIG. 4A illustrates a semiconductor structure in the form of a diode 400formed with a stepwise change in bulk-like materials. The diode 400 has,in order along a growth axis 410, a lower wurtzite metal layer 420 witha metal-polar growth including an n-type Al_(0.8) Ga_(0.2)N WBG emitter430, a gradient region in the form of an intrinsic Al_(x)Ga_(1−x)N alloy440 with a stepwise variation of composition along the growth axis thattransitions from the WBG wurtzite metal layer 420 to a NBG p-type GaNcontact layer 450 in discrete steps, and finally, an upper wurtzitemetal layer 460. The lower wurtzite metal layer 420 and the upperwurtzite metal layer 460 are effective ohmic metal contacts to form twoelectrical contacts for the diode 400.

FIG. 4B illustrates a bandgap diagram corresponding to the diode 400 ofFIG. 4A, showing how the bandgap transitions in steps 441 to 447 from aWBG material 432 to a NBG material 452. As will be appreciated, the WBGmaterial 432, gradient region 441-447, and NBG material 452 in FIG. 4Bcorrespond to the WBG emitter 430, stepwise graded alloy 440, and theNBG contact layer 450. The steps in the gradient region 441 to 447 maybe large or small but, for example, they could be graded with the firststep 441 being Al_(0.792)Ga_(0.208)N, second step 442 beingAl_(0.784)Ga_(0.218)N, stepping incrementally over many steps up to thesecond to last step 446 being Al_(0.0.016)Ga_(0.984)N, and the last step447 being Al_(0.007)Ga_(0.992)N.

FIG. 4C illustrates full spatial bandstructure of the diode 400illustrated in FIG. 4A showing the effect of a stepwise compositionalvariation of a WBG to a NBG transition provided by the stepwise gradedalloy 440. The depletion region is formed at the n-Al_(0.8)Ga_(0.2)Ninterface with the linearly chirped x_(ave)(z) region having an inducedp-type characteristic. FIG. 4D illustrates the conduction band variationof FIG. 4C, and FIG. 4E illustrates the valence band variation of FIG.4C.

FIG. 4E further shows the cross over in the energy ordering of the v=HH,LH and CH valence bands spatially in the structure. For application toLED function it is advantageous for transverse electric (TE) polarizedlight to be generated for light emission substantially perpendicular tothe plane of the layers. The high Al % portions with x(z)>0.65 would bedominated by the CH valence and thus be transverse magnetic (TM)polarized. This issue can be resolved by using superlattices to definethe effective alloy of the material. For example, using AlN and GaNlayers exclusively within a superlattice unit cell selects the TEoptical emission to be dominant for all values of the averagecomposition x_(ave).

FIG. 4F illustrates spatial variation of induced piezoelectric chargedensity due to the accommodation of lattice mismatch between differentAlGaN compositions. FIG. 4G illustrates spatial variation of inducedpyroelectric charge density due to the variation in the alloycomposition for the diode 400 of FIG. 4A. FIG. 4H illustrates electronand heavy-hole carrier concentrations generated within the diode 400 ofFIG. 4A.

It can be seen that the induced hole concentration within an otherwisenot-intentionally doped material is substantially larger than the smallintentionally doped contact layer of p-GaN. This in part solves a longstanding problem in the prior art wherein typically a semiconductor isrequired to be heavily doped to create a sufficiently low ohmic contactwith a metal contact electrode. Such a heavy doping density reduces thehost material quality and typically the carrier mobility and the crystalstructure are disadvantageously compromised. The induced doping regionof FIG. 4H shows a high activated hole density without the use ofsubstitutional dopants and thus represents an improved hole injector orreservoir which is not impeded by low hole mobilities or poor holetransport. Furthermore, the band diagram shows an induced depletionregion starting at about z=200 nm and ending at about z=300 nm, that ispositioned advantageously between the intentionally doped n-type WBGregion and the induced p-type region.

FIG. 5A illustrates a semiconductor structure in the form of a diode 500formed with a bilayered superlattice. In particular, the diode 500 has,in order along a growth axis 510, a lower wurtzite ohmic contact ormetal layer 520 with a metal-polar growth including a WBG emitter 530 inthe form of an n-type Al_(0.8)Ga_(0.2)N material, a gradient region inthe form of a bilayered superlattice 540 that transitions from the WBGemitter 530 to an NBG contact layer 550 formed of p-type GaN, and anupper wurtzite metal layer 560. The lower wurtzite metal layer 520 andthe upper wurtzite metal layer 560 are effective ohmic metal contacts toform two electrical contacts for the diode 500.

The bilayered superlattice 540 preferably comprises two dissimilarbinary compositions chosen from extreme III-N endpoints of AlN and GaN.Other combinations are also possible, for example Al_(x)Ga_(1−x)N/GaN orAl_(x)Ga_(1−x)N/AlN Al_(y)Ga_(1−y)N/Al_(x)Ga_(1−x)N where x≠y. It isalso possible to use three or more layers per unit cell, for exampletrilayered stacks of the form of AlN/Al_(x)Ga_(1−x)N/GaN. The use ofbinary constituent materials produces the largest areal charge sheetdensity at each heterojunction interface. Each bilayered period withinthe bilayered superlattice 540 has a fixed thickness of, for example 5nm (L_(GaN) of 1 nm and L_(AlN) of 4 nm) and varying composition suchthat it transitions from an [AlN/GaN] unit cell having an x_(ave) of0.8, adjacent the WBG emitter 530, to an x_(ave) of 0.01, adjacent theNBG contact layer. The unit cell thickness can be held constantthroughout and the ratio of the GaN and AlN thickness L_(GaN) andL_(AlN) selected to produce the desired x_(ave), where the unit cellbehaves as an equivalent bulk-like alloy of compositionAl_(xave)Ga_(=1−xave)N≡[L_(GaN)/L_(AlN)]_(xave).

FIG. 5B illustrates the magnitude of the spatial bandgap correspondingto the diode 500 of FIG. 5A, showing how the bandgap transitions over agradient region 542 from a WBG material 532 to a NBG material 552. Aswill be appreciated, the WBG material 532, gradient region 542, and NBGmaterial 552 in FIG. 5B correspond to the WBG emitter 530, bilayeredsuperlattice 540, and the NBG contact layer 550.

FIG. 5C illustrates a semiconductor structure in the form of a diode 501which is a variation of the diode 500 in FIG. 5A. The difference betweendiode 501 of FIG. 5C and diode 500 of FIG. 5A is that the WBG emitter530 of diode 500 is replaced with an n-type superlattice (n:SL) 531,preferably a SPSL, of x_(ave)=0.8. The n:SL 531 has a constant periodand is doped for n-type conductivity. Although illustrated as only a fewperiods, the n:SL 531 may comprise over, for example, 50 periods whilethe gradient region, i.e. the bilayered superlattice 540, may compriseover 1000 periods. FIG. 5D illustrates a full spatial bandstructure ofthe diode 501 illustrated in FIG. 5C showing the effect of the gradingof the bilayered superlattice 540. The conduction and valence band edgesare modulated along the growth axis with each heterojunction between theAlN and GaN layers. The n:SL 531 forms a depletion region between theinduced p-type region of the graded SL and is capped with a p-GaN layer.The i:SL graded region induces a hole density that is almost five timesgreater than can be achieved using a bulk-like composition grading.

Based on an understanding of how wurtzite III-N film polarity operateswith respect to heterojunctions and superlattices, preferred epitaxialstructures can be determined for specific polarity types. If a designgoal is to achieve an increased hole-carrier concentration by the use ofalloy or effective alloy composition grading, then the epitaxial growthsequence may be selected from one of a ‘p-UP’ or ‘p-DOWN’ design for ametal-polar or nitrogen-polar orientation, respectively.

FIG. 6 illustrates a metal-polar ‘p-UP’ LED structure 600 for ametal-polar film growth with respect to a growth axis 610 (sometimesreferred to as a growth direction ‘z’). To achieve an induced holeconcentration beyond that achievable with impurity doping alone, thecentre portion of the LED structure 600 has a gradient region 650 thattransitions from a WBG composition to a NBG composition with increasinggrowth along the growth axis 610, which is parallel to the spontaneouspolarization axis, in this case the c-axis of the wurtzite crystalstructure.

In order along the growth axis 610, the LED structure 600 comprises asubstrate 620, a buffer or dislocation filter region 630, an n-type WBGregion 640, the gradient region 650, and a NBG p-type region 660.Preferably, the substrate is a substantially transparent sapphire(Al₂O₃) substrate, for example, with a c-plane oriented sapphire (0001)surface or is a native III-N substrate, such as wurtzite AlN. Ohmicmetal contacts 670 and 672 are provided and an optical window 680 may beprovided to allow transmission of light from the top of LED structure600. It will be appreciated that light may instead, or additionally, betransmitted through the substrate 620. Furthermore, the buffer region630 may instead, or as well, be a dislocation filter region.

The n-type WBG region 640 is preferably in the form of a doped region asan n-type WBG layer or an n-doped superlattice of constant period andconstant effective alloy composition. The gradient region 650 can thenbe formed on the n-type WBG region 640 with an effective alloycomposition which varies as a function of distance along the growth axis610. The gradient region 650 forms the desired variation inbandstructure to form a transition from a WBG composition to a NBGcomposition. Optionally, at least a portion of the gradient region 650can be doped with an impurity dopant. For example, a p-type impuritydopant could be optionally integrated into the gradient region 650. Inpreferred forms the gradient region 650 comprises Al_(x(z))Ga_(1−x(z))or an [AlN/GaN] superlattice with a composition profile ‘k’ selected toachieve the spatial profile of the average alloy composition of eachunit cell given by:x_(ave)=x(z)=x_(WBG)−[x_(WBG)−x_(NBG)]*(z−z_(s))^(k), where z_(s) is thestart position of the grading.

The NBG p-type region 660 is deposited upon the gradient region 650,ideally having a similar effective alloy composition as the finalcomposition achieved by the gradient region 650. This mitigates apotential barrier being induced at a heterojunction interface betweenthe gradient region 650 and the NBG p-type region 660. In preferredforms the NBG p-type region 660 is a doped superlattice or bulk typeIII-N layer.

A cap layer, such as a p-GaN layer, can optionally be deposited as afinal cap layer to provide an improved ohmic contact and a source ofholes.

The optically transparency of the substrate 620 of the LED structure 600allows optical radiation generated from within the gradient region 650to advantageously propagate out of the device through the n-type WBGregion 640, through the buffer region 630, and finally out through thesubstrate 620 which has low absorptive losses. Light can also escapevertically out through the top of the structure 600, but the NBG p-typeregion 660 effectively filters shorter wavelengths of light and,accordingly, there can be an asymmetry in the wavelength response forlight output through the top and bottom of the LED structure 600. Lightgenerated from within the gradient region 650 can also escape laterallyas a ‘waveguided’ mode with a gradient refractive index, as a functionof the growth axis 610, further confining light to within the plane.

FIG. 7 illustrates a nitrogen-polar ‘p-DOWN’ LED structure 700 for anitrogen-polar film growth with respect to a growth axis 710. To achievean induced hole concentration beyond that achievable with impuritydoping alone, the centre portion of the LED structure 700 has a gradientregion 750 that transitions from a NBG composition to a WBG compositionwith increasing growth along the growth axis 710, which is substantiallyparallel to the spontaneous polarization axis, in this case the c-axisof the wurtzite crystal structure.

In order along the growth axis 710, the LED structure 700 comprises asubstrate 720 which is in the form of a substantially opaque substratesuch as Si(111) or a NBG native III-N substrate such as GaN, a bufferregion 730, a NBG p-type region 740, the gradient region 750, and an WBGn-type region 760. Ohmic metal contacts 770 and 772 are provided and anoptical window 780 may be provided to allow transmission of light fromthe top of LED structure 700. It will be appreciated that the bufferregion 730 may instead, or as well, be a dislocation filter region.

The NBG p-type region 740 is preferably in the form of a p-type NBGlayer or a p-doped superlattice of constant period and constanteffective or average alloy composition (with x_(ave)=NBG composition).The gradient region 750 is then formed on the NBG p-type region 740 withan effective alloy composition which varies as a function of growth axis710. The gradient region 750 forms the desired variation inbandstructure to form a transition from a NBG composition to a WBGcomposition. Optionally, at least a portion of the gradient region 750can be doped with an impurity dopant. In preferred forms the gradientregion 750 comprises Al_(x(z))Ga_(1−x(z)) or an [AlN/GaN] superlatticewith a composition profile ‘k’ ofx_(ave)=x(z)=x_(NBG)+[x_(WBG)−x_(NBG)]*(z−z_(s))^(k).

The WBG n-type region 760 is deposited upon the said gradient region750, ideally having a similar effective alloy composition as the finalcomposition achieved by the gradient region 750. This mitigates apotential barrier being induced at the heterojunction interface betweenthe gradient region 750 and the WBG n-type region 760. In preferredforms, the WBG region is a doped superlattice or bulk type III-N layer.

A cap layer, such as an n-Al_(x)Ga_(1−x)N (x≥0) layer, can optionally bedeposited to provide an improved ohmic contact and a source ofelectrons.

The LED structure 700 illustrated in FIG. 7 can be formed using opaquesubstrates 720, such as Si(111), which have a high absorptioncoefficient for optical wavelengths generated from within the gradientregion 750. Light can escape vertically through an optical outlet,preferably in the form of an aperture and/or window 780 in a suitableohmic contact material 772. Shorter wavelength light is preferentiallyabsorbed in the NBG regions creating further electron & holes throughre-absorption. It is anticipated that high quality p-GaN nativesubstrates or p-type SiC substrates can also be used.

Superlattice structures are preferably used to improve materialstructural crystal quality (lower defect density), improve electron andhole carrier transportation, and produce quantum effects that are onlyaccessible at such small length scales. Unlike bulk type III-Nmaterials, superlattices introduce new and advantageous physicalproperties, particularly in relation to diode and LED structures, suchas those illustrated in FIGS. 6 and 7. A homogeneous period superlatticecomprising at least two dissimilar semiconductor compositions, such asbilayered pairs of AlN and GaN, can be engineered to provide both (i)superlattice quantized miniband transport channels substantially alongthe growth axis (z), both in the tunnel barrier regime and above barrierregime; and (ii) improved carrier mobility within the plane of thesuperlattice layers by virtue of both periodicity inducted and bi-axialstrain induced band deformation so as to warp the energy-momentumdispersion. The superlattice can also mitigate strain accumulation bydepositing the constituent layers below their critical layer thickness.The superlattice having tailored conduction and valence band allowedenergies and spatial wavefunction probabilities can be manipulated bythe large built-in electric fields, such as the depletion fieldsdescribed herein. For example, a constant period SL can be grown toexhibit a highly coupled structure and generate an efficient carriertransport channel through the structure along the growth axis. Thehighly coupled nature of the partially delocalized wavefunctions can bereadily broken by large internal electric fields, rendering the coupledNBG regions essentially isolated (that is no communication betweenadjacent NBG regions). This can be advantageous for LED applications.

The superlattice quantized miniband transport channels improve transportalong the growth axis (z) and can be used to generate selective energyfilters. The improved carrier mobility can be used to dramaticallyreduce current crowding limitations in conventional device designscomprising mesa type structures. Conversely, the same superlatticestructure can be altered in operation by the being subjected to largeelectric fields, such as the depletion regions generated in thestructures disclosed herein.

Bulk III-N semiconductors can be characterised by a direct bandstructure which is defined by specific reference to the energy-momentumdispersion of the material which is dictated by the underlying atomicsymmetry. A direct bandgap III-N material is therefore a structure whichproduces simultaneously a lowest energy conduction band dispersion withminimum energy at zone centre k=0, as well as a highest lying valenceband dispersion, with its maximum also positioned at zone centre k=0.

Optical absorption and emission processes therefore occur as verticaltransitions in the energy-momentum space and primarily as first orderprocesses without phonon momentum conservation. The superlatticeperiodic potential, which is also on the length scale of the de Brogliewavelength, modulates the atomic crystal periodicity with a superposedsuperlattice potential which thereby modifies the energy-momentumbandstructure in a non-trivial way.

FIG. 8 illustrates estimated spatial band energy of a semi-infinitebilayered binary superlattice comprising a repeating unit cell of onemonolayer of GaN to 3 monolayers of AlN. The superlattice is shown withperiodic boundary conditions to simplify the calculation, and isstrained to a fully relaxed AlN buffer. FIG. 9 illustrates estimatedvalence band energy-momentum dispersion, with the quasi-delocalizedn_(SL)=1 HH, LH, and CH bands exhibiting highly warped departure fromparabolic dispersions used in bulk approximations. The effective massesof the valence band carriers, namely, the HH, LH and CH are therebymodified from their equivalents in bulk-like alloys. An important aspectof the superlattice as described is that the HH remains the dominantband for optical emission transitions between the lowest energyquantized conduction states and the lowest energy quantized HH states.Therefore, the superlattice preserves a TE character for 0≤x_(ave)<1,unlike for bulk-like Al_(x)Ga_(1−x)N where there is a transition in TEto TM for x˜0.65. This property is essential for vertically emissivedevices.

Short period superlattices with the period less than or equal to 10× thefree lattice constant of the constituent bulk materials form a newpseudo-alloy with pronounced differences in in-plane energy-momenta fromtheir equivalent bulk-like random metal distribution alloy counterparts.Furthermore, binary AlN/GaN superlattices form a new class of orderedalloys capable of producing new and improved properties over equivalentbulk-like alloys. Optical absorption and emission processes typicallyrequire accounting for the off-zone centre (k≠0) contributions of thesuperlattice band structure. For the present cases only the k=0 andlowest energy quantized and spatial wavefunction (labelled herein as then_(SL)=1 states) are used and are found experimentally to be sufficient.

Electric polarization fields can have an effect on the opticalproperties of chirped or intentionally profiled bandstructure. Forexample, consider a linearly chirped bilayered [AlN/GaN] superlattice,sandwiched between two oppositely positioned AlN cladding layers. FIGS.10A and 10B illustrate estimated spatial bandstructure of this notintentionally doped structure. Specifically, FIG. 10A illustrates zonecentre bandstructure with piezoelectric and pyroelectric fields absent,and FIG. 10B illustrates it with polarization fields applied generatingcomplex built-in electric fields along the growth axis (z). Theresulting built-in electric field along the growth axis (z) is solelydue to charges induced at each heterojunction due to pyroelectric(spontaneous) and piezoelectric effects. Each period of the superlatticeis held constant and the average alloy content with the i^(th) periodhaving thickness L^(i) _(AlN)(z) and L^(i) _(GaN)(z), such that Λ^(i)_(SL)=L^(i) _(AlN)(z)+L^(i) _(GaN)(z). The abrupt spatial modulation inthe conduction and heavy-hole valence band edges (i.e., at zone centerwavevector k=0) are indicative of atomically abrupt interfaces formed atthe heterojunction of each AlN and GaN transition. An atomically roughinterface would effectively broaden the potential wells but otherwiseresults in similar behaviour. In alternative embodiments, interfacialroughness at each heterojunction can be accounted for using anequivalent AlGaN interlayer, thus forming a trilayered unit cell.

FIGS. 11A and 11B illustrate the lowest energy calculated carrierspatial wavefunctions and quantized energy levels allowed within thestructure. Relatively thick AlN barriers, used in this example forclarity, show that the wavefunction tunnelling is significant into thebarrier for the lighter effective mass electrons compared to theheavy-holes. The general trend is for the quantized n=1 electron andhole wavefunction eigenenergies to drop further into the NBG potentialwell with increasing NBG material thickness.

The non-linear electric field generates a Quantum Confined Stark Effect(QCSE) across each GaN quantum well and an opposing QCSE across eachbarrier (AlN). The sign of the built-in electric field depends on thegrowth polarity of the material. The resulting wavefunction probabilitydensities confined within each potential energy minimum due to thebuilt-in fields are skewed spatially toward the lower potential energyinterface.

It can be seen that the electron and heavy-hole wavefunction maxima arespatially separated to opposing sides of the potential minimum and isexacerbated for larger GaN layer widths. This manifests as a reductionin the electron and HH wavefunction overlap for increasing GaN thicknessand creates a polarization induced transparency due to the reducedexciton oscillator strength. Conversely, thinner GaN layers improve then=1 conduction and HH wavefunction overlap and thus creates a higherprobability for an optical transition and increased emissionprobability. This effect is shown in FIGS. 12H & 13D.

FIG. 12A shows a stack 1200 for generating electrical and opticalportions of a p-n diode according to some embodiments. The stack 1200comprises a substrate SUB. The SUB is made of a material 1208 that isconducive to the forming of wurtzite III-N compositions having ametal-polar growth orientation along growth axis 1205. A n-type WBGbuffer layer (n:WBG) 1210 is deposited as a bulk-like alloy or as afixed average composition unit cell superlattice on the SUB. Next, ann-type SL (n:SL) is formed using average alloy content x_(ave_n) on then:SL. For example, the n:SL can be a 50 period SL formed with anx_(ave_n)=0.8. Preferably, the unit cell thicknesses 1211 and layerthicknesses are selected to form an n:SL that is substantiallytransparent (not absorbing) to a desired emission wavelength λ_(ex).

Next a graded SL (i:CSL) that is not intentionally impurity doped isformed. The i:CSL is used to induce a large hole concentration deepwithin the device that is free from substitutional impurity dopinglimitations. The i:CSL varies at least an average composition of a unitcell spatially along the growth axis from a WBG composition to a NBGcomposition. For example, the grading is selected to occur over 25 unitcells (i.e. 25 periods) with each unit cell total thickness 1212 heldconstant while the average Al % is varied, with the WBG compositionhaving x_(ave_CSL)=0.8 and the NBG composition having x_(ave_CSL)=0.0.An optional contact layer comprising p-GaN (p:NGB) is deposited upon thecompleted i:CSL. It is also possible to vary the unit cell thickness ofthe i:CSL as a function of the growth axis so long as the averagecomposition of the said unit cell follows the correct grading asdisclosed herein.

The i:CSL and the n:SL can be formed of bilayered unit cells comprisinga layer of GaN 1207 and a layer of AlN 1209. Other choices ofsuperlattice composition are also possible, and the composition of theunit cells can also be altered from period to period. For example, aunit cell period is selected to be equivalent to a combined thickness of2 ML of GaN and 4 ML of AlN. FIG. 12B shows the layer thickness of GaN1220 and AlN 1222 required to achieve an average alloy composition ofbilayered unit cell x_(ave). FIG. 12C also shows the average alloyvariation as a function of the growth axis 1205 for each of the n:SL andi:CSL. Curve 1223 shows a constant x_(ave_n)=0.8 is selected for then:SL whereas curve 1224 shows a linear x_(ave(z)) grading is selectedfor the i:CSL.

The induced spatial energy band structure of the stack 1200 is shown inFIG. 12D. The n:SL denoted by 1230 is intentionally doped with Siimpurities to a doping level of N_(D)=50×10¹⁸ cm⁻³. The i:CSL shows aninduced p-type portion 1233 as well as a depletion region 1232. Theportion of the i:CSL contacting the p:NGB 1234 shows the heavy-holevalence band pinned to the Fermi energy. Thus the n:SL/i:CSL/p:GaN diodeis formed with a further induced p-type region as shown further in FIG.12E.

FIG. 12E shows the spatial carrier densities along the growth axis. Thecarrier densities include the intentionally doped and resulting electronconcentration 1235 due to the n:SL, and the intentional p-GaN dopingconcentration 1239. Note the degeneracy of the valence band reduces theactivated doping density relative to the areal doping in the lattice.The portion of the i:CSL that has a large induced HH concentration 1237is shown along with the resulting depletion region 1236 defining then-i-p diode.

The lowest energy band edge quantized states are sufficient to determinethe majority of the electronic and optical character of the device.FIGS. 12F and 12G show the calculated n=1 states in the conduction andHH valence bands, respectively. Both the conduction and HH bands showminiband formation as indicated by the partially delocalizedwavefunctions 1242 and 1246 due to the short period n:SL. The depletionregion created by the induced p-type region of the i:CSL penetrates intoa portion of the n:SL and effectively breaλS the wavefunction couplingin regions 1241 and 1246. The confined electron and HH wavefunctions inregions 1241 and 1246 determine the recombination region of the deviceand thus the emission energy spectrum due to the direct transitionsbetween the n=1 conduction states and the n=1 HH states.

FIG. 12H shows the calculated spatial overlap integrals between all n=1conduction states with n=1 HH wavefunctions. The highest oscillatorstrength for an optical transition occurs in the region 1250, whereasthe portion of the i:CSL that has a wider GaN thickness creates onlyrelatively poor overlap 1255. This effect is highly advantageous forcreating polarization induced transparency within a p-like region. Theoptional p-GaN layer can also be removed to allow the higher energyphotons to be retroreflected back into the structure and outcoupledthrough the substrate. The full emission spectrum is shown FIG. 12Ishowing the strong excitonic emission peak 1256 due to the depletionregion created within the device and localized between the n:SL and thei:CSL. The smaller contributions 1258 are due to the i:CSL region.

FIG. 13A shows a stack 1300 for generating electrical and opticalportions of a p-i-n diode according to some embodiments. Thesuperlattices are again constructed from unit cells having binarywurtzite GaN 1207 and AlN 1209 layers and a metal-polar growth. However,stack 1300 comprises an additional i-type SL (i:SL) that is notintentionally doped. The i:SL is formed upon the n:SL. The i:SL is tunedspecifically to achieve an emission energy of light that issubstantially smaller in energy than that which the n:SL can absorb(i.e., the absorption edge of the n:SL is designed to have an energylarger than the emission energy of the i:SL). For example, the n:SL iscomposed of unit cells 1310 having 1 ML GaN and 2 ML AlN with 50repetitions. The i:SL is then selected to have an emission energy ofabout 246 nm by selecting a unit cell 1311 comprising 2 ML GaN and 4 MLAlN with 25 repetitions. However, more or less periods can be used inboth the n:SL and i:SL constructions.

Both the n:SL and i:SL have the same average alloy composition, namelyx_(ave_n)=⅔ and x_(ave_i)= 4/6=⅔ (i.e. x_(ave_n)=x_(ave_i)). Thuspolarization charges are balanced and do not induce p-type or n-typebehaviour. This is particularly advantageous for creating an improvedelectron and hole recombination region within the device. The graded SL(i:CSL) is formed with a unit cell that is varied from a WBG averagecomposition to a NBG average composition. The i:CSL unit cell thicknessis held approximately constant and is equivalent to a 3 ML GaN and 6 MLAlN unit cell. The thickness of the layers in each successive unit cellare altered in increments of ½ML thickness in order to achieve a desiredgrading profile of ⅔≤x_(ave_Csl(z))≤0 along the growth axis 1205. Thiscan be achieved with as little as 18 unit cells, but less or more unitcells can also be used.

FIG. 13B shows the spatial energy band structure within the n:SL 1310,i:SL 1312 and i:CSL 1314 along with the optional p-GaN region 1316. Thei:CSL induces a pinning of the HH valence band to the Fermi energy.

The induced carrier concentrations in the stack 1300 are shown in FIG.13C, where the large electron 1318 and HH 1322 carrier concentrationsare spatially generated. The intentional doping concentration in thep-GaN region 1326 is shown as well as the depletion region 1320 of thedevice.

FIG. 13D shows the calculated spatial conduction and HH overlapintegrals (i.e. oscillator strengths) for the exciton emission. Theexciton emission is clearly localized in a region 1330 that overlaps thei:SL. The polarization induced transparency region 1332 due to themajority of the i:CSL comprising NBG compositions does not significantlycontribute to the overlap integrals.

FIG. 13E shows the emission spectrum of the stack 1300 where the mainpeak 1338 is due to the i:SL and the smaller contributions 1340 are dueto the i:CSL region. The n:SL produces the feature labelled 1336 whichis typically suppressed due to phase space absorption/emission quenching(i.e., all states are fully occupied and cannot participate in opticalprocess due to phase-space absorption filling for in-plane wavevectorsk_(∥)˜0).

FIG. 14 illustrates an LED structure 1400 having a substrate 1420 whichis preferably a transparent substrate such as sapphire, a buffer and/ordislocation filter layer 1430, an n-type region 1440 in the form of ann-type superlattice (n:SL) of constant period and constant x_(ave), agradient region 1450 in the form of an i-type superlattice (i:SL), ap-type superlattice (p:SL) or bulk type contact region 1460, metalcontacts 1470 and 1472, and an optical window 1480.

Light λ_(L) can be emitted from the optical window 1480 and light λ_(S)can be emitted through the substrate 1420. Furthermore, light can escapethe structure via edge emission vectors λ_(E). For a linearly chirpedgradient region 1450 grown on a metal-polar orientation along the growthaxis (z) the gradient region 1450 would emit longer wavelength lightλ_(L) through the optical window, whereas shorter wavelength light λ_(S)would be emitted through the substrate. This is a direct result of the‘optical diode’ effect for emission of light within a spatially varyingeffective band gap region provided by the gradient region 1450, whichcan be particularly useful for DUV LED applications.

Another gradient pattern growth sequence is to vary period thickness asa function distance along the growth axis, while keeping the x_(ave) ofbilayered pairs constant. Such structures can be used to form tuneableoptical properties of an n-type and p-type region separately to therecombination within an i-type region. That is, by keeping x_(ave)constant, but varying the period of the superlattice, it is possible totune the optical properties of an LED stack of the form:

[n:SL x _(ave1),Λ₁]/[i:SL x _(ave2),Λ₂]/[p:SL x _(ave3),Λ₃]

where the effective Al % of each superlattice is held constantthroughout the p-n structure so thatx_(ave1)=x_(ave2)=x_(ave3)=constant, and is independent of growndirection (z). This case would not create an induced p-type or n-typeregion as average alloy composition is conserved.

The period of the superlattice repeating units cells, for example(Λ₁=Λ₃)<Λ₂ can be constructed so that x_(ave1)=x_(ave2)=x_(ave3) andthus the i:SL has a quantized energy transition between the n=1 electronand heavy-hole valence band that is smaller in energy than thecorresponding n=1 transition of at least one of the p:SL and n:SL. Theadvantage is the effective lattice matching of the in-plane latticeconstant of the superlattice unit cell (e.g., bilayered AlN/GaN pairs),which mitigates strain accumulation and reduces defect density due tomisfit dislocations.

An extension to the above example is a quasi continuous variation inperiod of the i:SL so as to form a linearly chirped band structuresuitable for carrier miniband injection and recombination to formbroadband luminescent devices. Consider the LED structure of FIG. 14showing an:

[n:SL x _(ave1)=const,Λ₁=const]/[i:SL x _(ave2)(z),Λ₂(z)]/[p-GaN]

The composition of the i:SL region is varied along the growth axis withaverage alloy composition controlled by the ratio of the thicknesses ofthe different composition layers comprising the unit cell. For the caseof two binary compositions of GaN and AlN the average Al mole fractionof the unit cell is defined herein as x_(ave)=L_(AlN)/(L_(GaN)+L_(AlN)),representing an equivalent bulk-like ordered alloy ofAl_(xave)Ga_(1−xave)N. The unit cell thickness from period-to-periodΛ_(SL)=(L_(GaN)+L_(AlN)) can also be varied. In such a case, the averagealloy composition of each unit cell conforms to the required gradient ortrend along the growth axis to achieve an induced n-type or p-typeregion or to balance the polarization and prevent band edge warping.

FIG. 15 illustrates a further gradient pattern growth sequence for agradient region 1500 with a chirped period and constant x_(ave)superlattice structure. Each of the sections (Λ¹ _(SL)−Λ⁴ _(SL))comprise N_(p)=25 repetitions with four sequentially stackedsuperlattices with incrementally varied period. The average alloycontent of each superlattice is kept constant. However, the period ofthe unit cell in each stack is varied by varying the thickness.

Many substrates have been explored for achieving wurtzitic III-Nepitaxy, namely, (i) native substrates and (ii) non-native substrates.At present bulk native GaN and bulk native AlN substrates exist,however, they are of extremely high cost and available only as smallwafer diameters which severely limits widespread penetration into highvolume applications such as, for example, LEDs and power transistors.

Non-native substrates are the most prevalent for III-N epitaxy and offerother advantages beyond simply cost reduction and large wafer diameters.The most popular non-native substrates for III-N epitaxy are sapphireand silicon. Many other non-native substrates exist such as, forexample, MgO, CaF2, and LiGaO.

Sapphire offers a compelling commercial and technological utility forhigh Al % III-N epitaxy due to the mechanical hardness, deep UV opticaltransparency, an extremely wide band gap, and its insulating properties.Sapphire is readily grown using bulk crystal growth methods such as CZand is manufacturable as extremely high quality structural qualitysingle crystal wafers, available in predominately, r-plane, c-plane,m-plane, and a-plane. C-plane sapphire is an important template surfacecompatible with III-N epitaxy.

Even though much work has been developed for wz-III-N/c-plane Al₂O₃,there still exists a large opportunity for further improving theepitaxial quality of III-N on these metal-oxide surfaces. Many attemptshave been demonstrated for semipolar and non-polar III-N epitaxy onr-plane, a-plane, and m-plane sapphire with limited improvement overthose found using hexagonal c-plane sapphire.

For the applications discussed herein, there is a preferred method forpreparing c-plane sapphire surface for achieving high qualitymetal-polar or nitrogen-polar III-N films. Sapphire, unlike wurtzite andzinc-blende crystals, has a more complex crystal structure. Sapphire isrepresented by a complex 12 unit cell comprising of oxygen planesinterposed with buckled bilayers of Al atoms. Furthermore, c-planesapphire exhibits a mechanical hardness much higher than r-planesapphire and thus polishing damage or polishing induced work hardeningcan readily impede production of atomically pristine surface species.Even though chemical cleaning can be used to produce a contaminant freesurface, and the bulk sapphire substrate shows excellent single crystalquality, the surface investigated by reflection high energy electrondiffraction (RHEED) exhibits a signature of c-plane sapphire which isalways indicative of an atomically rough and non-homogeneous surface.Surface steps in sapphire also readily expose mixed oxygen and atomiccrystalline regions which directly affect the initiating III-N polarityduring epitaxy, and typically results in polarity inversion domains(PIDs).

The first surface of the initiating template may be terminated in asubstantially atomically flat and homogeneous surface terminationspecies. For example, a bulk Si(111) oriented surface enablesimprovements in epitaxial polarity control by virtue of the homogeneoussubstrate composition, namely, Si atoms. By careful initial epitaxialfilm deposition to the Si surface it is possible to induce eitherAl-polar or N-polar AlN epitaxial growth.

FIG. 16 illustrates an intentionally flipped, but otherwise laterallyhomogeneous, polarity type of a III-N complex structure comprising anitrogen polar region 1600, a polarity flip plane 1620, and a metalpolar region 1640. The total structure may be engineered to contain aplurality of laterally disposed regions within the epitaxial growthsequence of substantially different polarity-type slabs. That is, afirst polarity wz-III-N region is grown upon an initiating template.Then the final surface of the first polarity region is modified orengineered to result in an opposite polarity-type region for a secondpolarity wz-III-N region. A plurality of polarization-type regions canthus be formed by effectively flipping the polarity of each of the III-Ndistinctive slabs.

Polarity-type inversion of a final wz-III-N region surface is possibleusing a heavily saturated surface coverage of a surfactant type adatom.Geometric frustration is used to advantageously reconstruct theresulting surface which is favourable for achieving the desiredpolarity-type for the subsequently deposited III-N surface. Suchpolarity inversion of multilayered epitaxial structures exhibitinghomogeneous polarity-type within a 2D III-N slab are advantageous forcreating new device structures with improved performance overunipolarity-type epitaxial devices. For example, polarity flipping ofsurface layers can be used advantageously to lower Schottky barrierlimitation of metal ohmic contacts to polar wz-III-N materials. Thepolarity-type flipped bilayer acts as a degenerately doped tunneljunction and improves the performance of III-N devices.

Polarization-type flipping structures can be extended to more complexstructures forming inversion modulated structures which may further beperiodic. Such structures can be used to either enhance the polarproperties of devices or substantially reduce the in-built polarizationfields. This presents a new method for producing non-polar materialsusing wurtzite films grown along the c-axis.

FIG. 17 illustrates a broad flow diagram for forming semiconductorstructures having a gradient region. First, a gradient pattern growthsequence is selected (step 10), then an appropriate substrate isselected (step 20), and finally the selected gradient pattern is formedon the substrate (step 30). The gradient pattern growth sequence isselected (step 10) such that it transitions from a WBG to a NBG or froma NBG to a WBG material along the grown axis (z). Additional layers,such as a buffer or dislocation filter region, may also be growndepending on the desired structure.

FIG. 18A illustrates a semiconductor structure 1800 having an optionalp-type GaN region in the form of a p-GaN contact region 1820, a p-typesuperlattice (p:SL) region 1840, an i-type superlattice (i:SL) region1860, and an n-type superlattice (n:SL) region 1880. Each of the p:SLregion 1840, the i:SL region 1860 and/or the n:SL region 1880 can be inthe form of a SPSL.

The heterointerface between the i:SL region 1860 and the p:SL region1840 or the p-GaN contact region 1820 is of particular concern becausethe electron mobility and injection efficiency is much higher than forholes, resulting in electron overshoot through the i:SL region 1860 andhence higher recombination near the i-p interface. This is confirmedexperimentally by an optical emission feature at about 360 nm.Additionally, the high concentration of Mg dopants in the p:SL region1840 can also act as non-radiative recombination sites. It has beenfound to be beneficial to shift the recombination towards the centre ofthe active region away from all non-radiative recombination sites bychoosing specific superlattice compositions and grading/chirpingsuperlattices to use the polarisation charges to modify thebandstructure.

Furthermore, electron penetration in prior art LED devices based onmajority bulk-like and quantum well constructions is high, and istypically reduced by the introduction of electron blocking potentialbarriers on the p-side of the device. Electron blocking in the presentmethod is achieved automatically by the conduction minibands andsuperstates above the AlN conduction edge due to the superlatticepotential. The superlattice potential acts as an electron energy filterfor transport across the device along the growth axis.

FIG. 18B illustrates an energy band structure for a semiconductor device1800. The spatial energy band diagrams of FIGS. 18B-H represent thesuperlattice regions as their equivalent n=1 quantized eigenenergytransition and thus represents an equivalent ordered alloy of the SL.

Complex semiconductor structures formed of two or more contiguoussemiconductor structures and/or semiconductor superlattices have beendescribed above. In some embodiments, a first of the two or morecontiguous semiconductor structures and/or semiconductor superlatticescan have a larger change in composition along the growth axis and asecond of the two or more contiguous semiconductor structures and/orsemiconductor superlattices can have a smaller change in compositionalong the growth axis. For example, the first of the two or morecontiguous semiconductor structures and/or semiconductor superlatticesinduces a heavy p-type conductivity, and the second of the two or morecontiguous semiconductor structures and/or semiconductor superlatticesinduces a light p-type conductivity

FIG. 18B shows the p:SL region 1840 is chirped or graded with a largechange in composition (e.g. from x_(ave)=0.6 to 0) that results in heavyp-type polarisation doping of the entire p:SL region 1840, the i:SLregion 1860 is chirped such that the composition decreases from bottomto top (e.g. from a composition x_(ave)=0.66 to 0.6) that induces lightp-type bulk polarisation doping over the intrinsic region to compensatefor low hole injection efficiency; and the n:SL region 1880 has a highAl content (e.g. a 1 ML GaN:2 ML AlN SL with a uniform composition ofx_(ave)=0.66).

There are no abrupt changes in composition at any of the interfacesbetween regions which eliminates any polarisation induced sheet charges,eliminates barriers due to band offsets, and can also result in higherquality interfaces since there are no abrupt changes in latticeconstant. The polarisation doping density in the p:SL and i:SL regionscan be changed either by changing the total thickness of the region, orby changing the composition at their interface. For example, changingthe composition at the i:SL/p:SL interface to 0.5 (from 0.6) willincrease the p-type doping in the i:SL region and decrease it in thep:SL region. Decreasing the thickness of the p:SL region (to 25 nm forexample) will increase the doping density in the p:SL region withoutchanging the i:SL region.

FIG. 18C illustrates an energy band structure for a semiconductor device1800 wherein the p:SL region 1840 is uniform (e.g. x_(ave)=0.66), thei:SL region 1860 is uniform (e.g. x_(ave)=0.66), and the n:SL region1880 is uniform (e.g. x_(ave)=0.66). Since all the superlattice regionshave the same composition there are no polarisation effects, except forat the p:SL/p-GaN interface. A benefit of this design is that the p:SLand i:SL regions are lattice matched (i.e., the in-plane latticeconstants of the unit cells comprising the strained layers are equal)and thus there are fewer defects at this interface to act asnon-radiative recombination sites. Compared with strong p-typepolarisation doping at the p:SL/p-GaN interface, the doping in the p:SLregion has relatively little effect, as shown in FIG. 18C.

FIG. 18D illustrates an energy band structure for a semiconductor device1800 wherein the p:SL region 1840 is uniform (e.g. x_(ave)=0.2), thei:SL region 1860 is uniform (e.g. x_(ave)=0.66), and the n:SL region1880 is uniform (e.g. x_(ave)=0.66). The composition of the p:SL region1840 is lower than the i:SL region 1860 (e.g. a 2 ML GaN:4 ML AlN i:SLand 6 ML GaN:2 ML AlN p:SL), resulting in p-type polarisation doping ofthe i:SL/p:SL and p:SL/p-GaN interfaces which pins the valence bandabove the Fermi energy level at either side of the p:SL region 1840 asshown in FIG. 18D. This causes a hole reservoir to form at the i:SL/p:SLinterface. There is still some change in the in-plane lattice constantbetween these unit cells, unlike in the form illustrated in FIG. 18C,but the p-type polarisation doping of the p:SL region 1840 can be morebeneficial than a fully lattice matched p:SL region 1840. The case abovewill have the p:SL in a state of compression.

FIG. 18E illustrates an energy band structure for a semiconductor device1800 wherein the p:SL region 1840 is chirped (e.g. x_(ave)=0.66-0), thei:SL region 1860 is uniform (e.g. x_(ave)=0.66), and the n:SL region1880 is uniform (e.g. x_(ave)=0.66). Chirping or grading of the p:SLregion 1840 between the i:SL 1860 region and the p-type GaN region 1820causes bulk p-type polarisation doping of the p:SL region 1840, ratherthan sheet charges at each of the interfaces which increase the p-typeconductivity through the p:SL region 1840 and improve the holeinjection. It also has the benefit of eliminating band discontinuitiesat the i:S L/p:SL and p:SL/p:GaN interfaces which further increases holeinjection efficiency. This can reduce the dislocation density at thei:SL/p:SL heterointerface, though the entire p:SL region 1840 sincethere is no abrupt change in lattice constant.

P-type polarisation doping in the p:SL region 1840 is very high (˜5×10¹⁸cm⁻³) and that the bandstructure and hole concentrations are almostidentical whether the p:SL region 1840 is intentionally doped with Mg ornot. Thus, a variation on this design removes the intentional Mg dopingin the chirped p:SL region 1840 and it is grown essentially as anintrinsic or not-intentionally doped region. To avoid confusion, thisregion is called an induced p:SL region since it is still polarisationdoped p-type. The polarisation induced doping density is dependent onthe change in composition and the distance over which the region isgraded. So, if the composition change is fixed by the regions on eitherside, then the doping density can be increased by reducing the thicknessof the graded region. This design has the benefit of removing the Mgimpurity dopants from near the recombination region which can increasemobility and reduce non-radiative recombination. In general, Mg dopingof a p:SL does not achieve as high structural quality as n:SL and i:SL,since the p:SL must be grown nitrogen-rich to allow Mg dopants toincorporate substitutionally and results in atomically rough layers. Ifthe p:SL region 1840 can be grown without requiring Mg then itsstructural quality can be improved and thus increase advantageously thedesired device performance.

FIG. 18F illustrates an energy band structure for a semiconductor device1800 wherein the p:SL region 1840 is uniform (e.g. x_(ave)=0.6), thei:SL region 1860 is uniform (e.g. x_(ave)=0.6), and the n:SL region 1880is uniform (e.g. x_(ave)=0.66). It is compared with a lattice matchedstructure as illustrated in FIG. 18C. The i:SL region 1860 compositionis chosen to be lower than the n:SL region 1880. The lower i:SL region1860 composition causes p-type polarisation doping of the n:SL/i:SLinterface which raises the energy bands in the intrinsic region andincreases the intrinsic region hole concentration as shown in FIG. 18F.This is a simplification of chirped i:SL region structures but achievesa similar result of increasing the hole concentration in the intrinsicregion. This enables a simpler growth as the layers are all uniform.

FIGS. 18G and 18H illustrate band energy structures for a semiconductordevice 1800 wherein the p:SL region 1840 is chirped (e.g.x_(ave)=0.5-0.1), the i:SL region 1860 is chirped (e.g.x_(ave)=0.7-0.6), and the n:SL region 1880 is uniform (e.g. x=0.66).FIG. 18G has 2D polarisation sheet charges intentionally introduced atthe n:SL/i:SL interface and FIG. 18H 2D polarisation sheet chargesintentionally introduced at the i:SL/p:SL interface. A small compositionchange at one or more of the interfaces is introduced to induce sheetpolarisation charges. For example, if the top of the i:SL region 1860has a composition of 0.6 and the bottom of the p:SL region 1840 has acomposition of 0.5, the interface will be p-type polarisation dopedwhich can induce a two dimensional hole gas (2DHG). Likewise, if thebottom of the i:SL region 1860 has a composition of 0.7 on a 66% n:SLregion 1880 (i.e. x_(ave)=0.66) a small n-type sheet charge will beinduced. This heavy sheet doping can be useful to provide reservoirs ofcarriers to improve injection efficiency and to reduce carrierovershoot. It can also improve current spreading due to high lateralmobility in the 2DHG.

Other variations of the semiconductor structure 1800 can be implementedas well. For example, a uniform p:SL region 1840 can be grown and onlythe i:SL region chirped to lightly induce p-type polarisation, forexample from x_(ave)=0.66 to x_(ave)=0.55. A chirp in the oppositedirection (i.e. from high Ga content to low Ga content) can inducen-type polarisation doping instead of p-type. This may be used at thetop of the n:SL region 1880 to provide a very heavily doped layer to actas an electron reservoir. An n-type polarisation chirp may also beuseful to heavy dope a layer buried in the n:SL region 1880 for alateral current spreading layer, or to provide a highly doped region forohmic contact formation.

The p-GaN is considered optional, and contact can be directly to thep:SL region 1840. This can significantly increase the light extractionefficiency if the p:SL region 1840 is chosen to be transparent at theoperating wavelength and the p-contact is reflective. For chirped p:SL,the grading of the average alloy composition can be simply terminated ata composition which is still sufficiently transparent, for examplex_(ave)=0.4, and contacted directly. However, this could reduce thecomposition range over which the chirp can be performed and thus reducethe potential polarisation doping.

The invention advantageously provides semiconductor structures that havebroad applications, particularly in relation to DUV LEDs. For example,the invention advantageously overcomes, or at least reduces, many of theconstraints that limit commercial development of DUV LEDs.

Although the invention has primarily been described with respect todiodes, and LEDs which are a preferred embodiment of the invention, itwill be appreciated that, where the context permits, other semiconductorstructures and devices could be constructed.

In this specification, the term “superlattice” refers to a layeredstructure comprising a plurality of repeating unit cells including twoor more layers, where the thickness of the layers in the unit cells issmall enough that there is significant wavefunction penetration betweencorresponding layers of adjacent unit cells such that quantum tunnellingof electrons and/or holes can readily occur.

In this specification, adjectives such as first and second, left andright, top and bottom, and the like may be used solely to distinguishone element or action from another element or action without necessarilyrequiring or implying any actual such relationship or order. Where thecontext permits, reference to an integer or a component or step (or thelike) is not to be interpreted as being limited to only one of thatinteger, component, or step, but rather could be one or more of thatinteger, component, or step etc.

The above description of various embodiments of the present invention isprovided for purposes of description to one of ordinary skill in therelated art. It is not intended to be exhaustive or to limit theinvention to a single disclosed embodiment. As mentioned above, numerousalternatives and variations to the present invention will be apparent tothose skilled in the art of the above teaching. Accordingly, while somealternative embodiments have been discussed specifically, otherembodiments will be apparent or relatively easily developed by those ofordinary skill in the art. The invention is intended to embrace allalternatives, modifications, and variations of the present inventionthat have been discussed herein, and other embodiments that fall withinthe spirit and scope of the above described invention.

In this specification, the terms ‘comprises’, ‘comprising’, ‘includes’,‘including’, or similar terms are intended to mean a non-exclusiveinclusion, such that a method, system or apparatus that comprises a listof elements does not include those elements solely, but may well includeother elements not listed.

The reference to any prior art in this specification is not, and shouldnot be taken as, an acknowledgement or any form of suggestion that theprior art forms part of the common general knowledge.

What is claimed is:
 1. A method of forming a semiconductor structure,comprising: growing along a growth axis a first superlattice having aplurality of first unit cells and a first polar crystal structure, thegrowth axis being substantially parallel to a spontaneous polarizationaxis of the first polar crystal structure; growing along the growth axisa second superlattice contiguous with the first superlattice, whereinthe second superlattice has a plurality of second unit cells and asecond polar crystal structure, the growth axis being substantiallyparallel to a spontaneous polarization axis of the second polar crystalstructure; changing an average composition of the second unit cells ofthe second superlattice monotonically from a first average compositioncorresponding to a first wider band gap (WBG) material to a secondaverage composition corresponding to a first narrower band gap (NBG)material or from a third average composition corresponding to a secondNBG material to a fourth average composition corresponding to a secondWBG material along the growth axis to induce p-type or n-typeconductivity; and growing along the growth axis a third superlatticecontiguous with the second superlattice, wherein the third superlatticehas a plurality of third unit cells and a third polar crystal structure,the growth axis being substantially parallel to a spontaneouspolarization axis of the third polar crystal structure; wherein thefirst, second and third unit cells of the first, second and thirdsuperlattices respectively each comprise a plurality of at least twodistinct layers formed of substantially single crystal semiconductors.2. The method of claim 1, wherein the p-type conductivity of the secondsuperlattice is induced by: growing the second superlattice with acation-polar crystal structure and changing the average composition ofthe second unit cells in the second superlattice monotonically from thefirst average composition to the second average composition along thegrowth axis; or growing the second superlattice with an anion-polarcrystal structure and changing the average composition of the secondunit cells in the second superlattice monotonically from the thirdaverage composition to the fourth average composition corresponding to aWBG material along the growth axis.
 3. The method of claim 2, wherein:the anion-polar crystal structure is a nitrogen-polar crystal structureor an oxygen-polar crystal structure; and the cation-polar crystalstructure is a metal-polar crystal structure.
 4. The method of claim 1,wherein the n-type conductivity of the second superlattice is inducedby: growing the second superlattice with a cation-polar crystalstructure and changing the average composition of the second unit cellsin the second superlattice monotonically from the third averagecomposition to the fourth average composition along the growth axis; orgrowing the second superlattice with an anion-polar crystal structureand changing the average composition of the second unit cells in thesecond superlattice monotonically from the first average composition tothe second average composition along the growth axis.
 5. The method ofclaim 4, wherein: the cation-polar crystal structure is a metal-polarcrystal structure; and the anion-polar crystal structure is anitrogen-polar crystal structure or an oxygen-polar crystal structure.6. The method of claim 5, wherein: the first superlattice is dopedn-type and the third superlattice is doped p-type, or the firstsuperlattice is doped p-type and the third superlattice is doped n-type;and impurity dopants are included in one or more of the at least twodistinct layers of the first and second unit cells of the first andthird superlattices respectively.
 7. The method of claim 5, furthercomprising: forming ohmic metal contacts to the semiconductor structure;and forming an optical window to form a light emitting diode (LED)device.
 8. The method of claim 1, wherein the average composition of thesecond unit cells in the second superlattice is changed in a stepwisemanner along the growth axis.
 9. The method of claim 1, wherein theaverage composition of the second unit cells in the second superlatticeis changed by changing a thickness of one or more of the at least twodistinct layers of the second unit cells in the second superlattice. 10.The method of claim 1, wherein a thickness of the second unit cells inthe second superlattice is constant along the growth axis.
 11. Themethod of claim 1, wherein a composition of one or more of the at leasttwo distinct layers of the first, second or third unit cells in thefirst superlattice, the second superlattice, or the third superlatticerespectively is selected from the following: gallium nitride (GaN);aluminium nitride (AlN); aluminium gallium nitride (Al_(x)Ga_(1−x)N)where 0≤x≤1; boron aluminium nitride B_(x)Al_(1−x)N where 0≤x≤1; andaluminium gallium indium nitride (Al_(x)Ga_(y−x−y)N) where 0≤x≤1, 0≤y≤1and 0≤(x+y)≤1.
 12. The method of claim 1, wherein a composition of oneor more of the at least two distinct layers of the first, second orthird unit cells in the first superlattice, the second superlattice, orthe third superlattice is selected from the following: magnesium oxide(MgO); zinc oxide (ZnO); and magnesium zinc oxide (Mg_(x)Zn_(1−x)O)where 0≤x≤1.
 13. The method of claim 1, wherein the at least twodistinct layers of each unit cell each have a thickness that is lessthan a de Broglie wavelength of a charge carrier in the respectivelayer.
 14. The method of claim 1, wherein the at least two distinctlayers of each unit cell each have a thickness that is less than orequal to a critical layer thickness required to maintain elastic strain.15. The method of claim 1, wherein: impurity dopants are included in oneor more of the at least two distinct layers of each of the second unitcells of the second superlattice to enhance the induced p-type or n-typeconductivity.
 16. The method of claim 1, further comprising flipping apolarity-type of material between the first superlattice and the secondsuperlattice, or between the second superlattice and the thirdsuperlattice.
 17. The method of claim 1, wherein a buffer or dislocationfilter region is grown on a substrate preceding the first superlattice.18. The method of claim 17, wherein: the substrate is selected as asapphire (Al₂O₃) substrate, an aluminium nitride (AlN) substrate, asilicon substrate, or a gallium nitride (GaN) substrate.